@darootler kc-60 is a duplicate of the zcp ticket, both are now closed. As part of this ticket the error message you now see was implemented to solve a crash at startup.

Since this then potentially concerns search folders, rebuilding these (which is not the same of deleting the index of kopano-search) might help.

The man page of kopano-server states the following:

--restart-searches, -R
Rebuild all search folders. This may take some time and is only needed when your search folders have become out-of-sync with the actual data in the
database. The sync will start synchronously at the start of the server, and you will have to wait for all searches to complete before connecting to the
server.